The described subject matter relates generally to combustion engines and more specifically to methods for joining components for combustion engines.
Combustion engines including internal combustion engines and gas turbine engines such as turbofans, turboshafts, and turboprops provide motive power in a wide variety of industries and applications. Ground-based combustion engines such as internal combustion engines and gas or steam turbines are also used for generating electrical and/or mechanical power. Advances in material compositions and processing have led to the use of more exotic materials in an effort to improve engine efficiency. A more refractory (e.g., more thermally resistant) material could be used to insulate a less refractory material. Thermal-resistance properties generally relate to resistance of a substrate to thermally induced phase changes.
Since a more refractory material is typically heavier, more expensive, and/or lacking in a key property (e.g., ductility) than less thermally resistant materials, it would be helpful to use a less refractory material where there is less risk of exposure of that material to extreme conditions. However, there have historically been at least two issues with this approach. First, there is often a mismatch in the coefficient of thermal expansion (CTE) between materials. If the mismatch is too large, it increases thermally induced strains and the risk of premature failure at the material interface. Second, a suitable, more refractory material may still have a relatively high thermal conductivity, and does not adequately insulate the otherwise suitable less refractory material.
The issues of differential CTE and high thermal conductivity arise, among other places, in the hot section of turbine engines. For example, combustor and turbine components are exposed to hot working gases and thus are often manufactured from combinations of specialized superalloys, ceramics, and/or composites. Turbine blades and combustor parts often require vapor or thin film deposition of a metallic bond layer to form a suitable interface between a less refractory superalloy substrate and a more refractory ceramic coating. The metallic bond layer mediates the different CTEs of the superalloy substrate and ceramic coating, while also controlling conduction of heat into the superalloy substrate. Despite a mediating metallic layer, substantial practical limitations remain on usable combinations of superalloy and/or refractory ceramic substrates in other applications.
To reduce weight and improve efficiency, it would be helpful to be able to utilize the best and most cost-effective materials in all parts of the engine. This would require a number of dissimilar materials to be in close proximity to each other. However, each material is likely to have different thermal and mechanical responses. Thus designers must be extremely careful about which materials can be used together, and particularly about combinations of materials which are to be physically joined or fastened together.